Z. Cvijović

Prediction of ductile fracture initiation using micromechanical analysis

Abstract In the paper ductile fracture initiation analysis of low-alloyed ferritic steel has been made by application of two micromechanical models: the Rice–Tracey void growth model and the Gurson–Tvergaard–Needleman (GTN) model. The aim of the study was to analyse transferability of micromechanical parameters determined on specimens without initial crack to pre-cracked specimens. A significant part of the research has been carried out through participation in the round robin project organised by the European Structural Integrity Society (ESIS). Tensile tests have been performed on cylindrical smooth specimens and CT specimens. Critical values of micromechanical parameters determined on smooth specimen for both applied models, have been used for prediction of the crack growth initiation in CT specimen. Modelling of the first phase of ductile fracture––void nucleation––has been carried out using quantitative metallographic analysis of non-metallic inclusion content in tested steel. For determination of critical values of model parameters corresponding to ductile fracture initiation a simple procedure has been applied based on a combination of experimental and numerical results. Evaluated J -integral values corresponding to onset of crack growth, J i , are in good agreement with experimental result and both models have proved to be suitable for determination of the ductile fracture initiation in tested steel. The effect of FE size at a crack tip on J i -value has been particularly analysed: it has been established that the calculation with FE size corresponding to the mean free path λ between inclusions in steel gives results that are in accordance with the experimental ones.

Crack Growth Resistance of Overaged Al-Zn-Mg-Cu Alloys

The crack growth resistance of the Al-Zn-Mg-Cu alloy forgings in overaged condition was investigated with three industrially produced alloys, which showed differences in the microstructures governed by compositional variations. Fatigue-crack propagation experiments were conducted at ambient temperature and variations in crack growth rates (da/dN) as a function of applied stress intensity range (ΔK) were related to the characteristics of microstructures, including coarse intermetallic (IM) particles and precipitates. It appears that the crack growth rate increases systematically with an increase of the impurity level, which in turn increases the amount and size of large Fe- and Si-containing IM particles while decreases their spacing. That degradation in resistance to crack growth was attributed to the acceleration of the crack initiation and propagation by coarse IM particles were confirmed by in-situ SEM observation of the fracture process. The observed anisotropy in fatigue behavior was caused by the anisotropy in coarse IM particle orientation.

Fracture Toughness Modeling in High-Strength Al-Based Alloys

The fracture toughness of the commercial 7xxx alloy in T73 overaged temper is modeled to determine the influence of the microstructural parameters associated with the coarse constituent particles on the fracture process. To develop a quantitative relationship between the plane strain fracture toughness, KIc, and the microstructural attributes, an extensive stereological analysis and mechanical tests are performed. Fracture behavior of heat-treated forgings is characterized as a function of Fe and Si impurity levels. The data are compared to current fracture toughness models, with the results applied to improve the modeling of toughness using microstructural parameters and basic tensile properties. The appropriate modification of the existing model is proposed and a multiple micromechanism-based model is developed. This required the measure of parameters representing the relative contributions of different fracture micromechanisms to the fracture surface morphology.

Fractographic analysis of fatigue damage in 7000aluminium alloys

In this paper, an attempt is made to correlate the fatigue damage in 7000 aluminium alloys with different impurity contents to the microstructural features and to explain their interdependence through fractographic observations. The Paris constants of these alloys in the form of hot‐forged plates subjected to the overaged T73 temper are evaluated and differences in the fatigue crack growth rate described by striation spacing measurements. Scanning electron microscopy analysis of fatigue fracture surfaces revealed that the type and morphological parameters of coarse intermetallic particles play a critical role in fatigue crack growth behaviour. The elemental distribution determined by means of energy‐dispersive spectroscopy analysis showed that the fractured particles accelerating the crack advances are larger particles of Fe‐rich phases. The fatigue crack growth rate increases considerably with increasing amounts of these particles. The smaller η, S and Mg2Si particles contribute beneficially to fatigue life.

Ductile fracture prediction of steam pipeline steel

Considering the conditions to which steels used for the manufacture of steam pipelines are exposed, the micromechanism of their destruction in exploitation is exclusively the ductile one. In order to make an estimation of the level of the damage that occurs in exploitation, in this paper a combined experimental and numerical procedure has been developed based on micromechanical or local approach to the fracture mechanics of metallic materials. After the analysis of the results obtained for micromechanical criterion of failure for virgin steel and that used in the steam pipeline, a proposal for prolongation of the working life of tested steel for steam pipelines until the next overhaul is given.

The Influence of Microstructure on Ductile Fracture Initiation in Low-Alloyed Steel

In this paper the quantitative microstructural analysis of low-alloyed pressure vessel steel has been discussed. The values of volume fraction and mean free path of non-metallic inclusions determined in that way were used in the numerical analysis. A series of elastic-plastic calculations of round smooth specimen and compact tension (CT) specimen was made using finite elements (FE) method under conditions of ductile fracture initiation of tested steel, followed by large plastic deformation. The influence of microstructure of tested ferritic steel on crack initiation and growth was considered using two micromechanical models.

Structural Integrity Assessment by Local Approach to Fracture

Local approach to fracture has been developed for complete understanding of fracture mechanism [1,2], including the material degradation process. This approach combines theoretical, experimental and numerical solutions, enabling less conservative assessment of crack significance and residual stress.

Micromechanical Coupled Study of Crack Growth Initiation Criterion in Pressure Vessel Steel

We present results of the combined design-theoretical investigation of the mechanism of crack growth at the onset of ductile fracture of NPP reactor pressure vessels. Micromechanical approach to the prediction of ductile fracture has been applied, according to which the volume fraction of voids in the deformed material is determined by the finite-element method. On the basis of CT-specimen tests and known damage parameters, determined for smooth spherical specimens, we propose a micromechanical criterion of crack growth initiation for ductile fracture.

ANALYSIS OF THE TRANSFERABILITY OF MICROMECHANICAL PARAMETERS OF DAMAGE OF STEEL UNDER THE CONDITIONS OF DUCTILE-FRACTURE INITIATION

For the description of the mechanical behavior of a cracked material, we use a macromechanical model of fracture based on the well-known criterion of plastic yield for porous materials. This model is realized as a computational scheme with the help of the finite-element method. The critical values of the micromechanical parameters of damage are experimentally measured and numerically evaluated for smooth specimens with an aim to predict the onset of plastic yield in a compact cracked specimen. For a low-alloy pressure-vessel steel considered as an example, we show that the proposed micromechanical model can be used for the prediction of the onset of fracture processes in both smooth and compact specimens.

Determination of Parameters of Local Approach to Ductile Fracture of Steel Using Combined Experimental-Numerical Procedure

In this paper, two examples of combined experimental-numerical procedure of local approach to fracture mechanics have been presented. In first example, the level of fracture of steel in exploitation - pressurised steamline at elevated temperature - has been determined using uncoupled Rice-Tracey [1] model of local approach.